This invention relates to a magnetic storage device, a writing method for a magnetic storage device and a manufacturing method for a magnetic storage device, and more particularly to a nonvolatile magnetic storage device that stores information by making use of a phenomenon wherein the resistance value varies depending upon whether the spin direction of a ferromagnetic substance is in a parallel direction or an anti-parallel direction, a writing method for a magnetic storage device and a manufacturing method for a magnetic storage device.
With the rapid progress of information communication equipments, particularly of small size equipments for personal use, such as portable terminals, further progress in performance, such as increases in integration, increases in speed and reduction in power consumption, is demanded for component devices of communication equipments, such as memory devices and logic devices. Particularly, nonvolatile memory is considered to be an essential device required in the age of ubiquitous computing and communications.
For example, even if the power supply is used up or suffers from some trouble or a server and a network are disconnected from each other by some trouble, nonvolatile memory can protect important information. Thus, increases in the density and the capacity of a nonvolatile memory becomes progressively significant as a technique for replacing a hard disk or an optical disk with which miniaturization is essentially impossible due to the presence of a movable part.
Further, while recent portable equipments have been designed such that unnecessary circuit blocks are placed into a standby state to suppress the power consumption as far as possible, if a nonvolatile memory which can serve both as a high speed network memory and a large capacity storage memory can be implemented, then the power consumption and non-productive use of the memory capacity can be minimized. Also an instant-on function which a device can start up instantly when the power supply is made available, can be implemented if a high-speed large-capacity nonvolatile memory can be implemented.
As nonvolatile memories, a flash memory which uses a semiconductor and an FRAM (Ferroelectric Random Access Memory) which uses a ferroelectric substance are available. However, flash memory has a drawback that the writing speed is as low as the microsecond order. On the other hand, FRAM has a drawback that the number of times by which it can be rewritten is 1,012 to 1,014 and the durability is not sufficiently high to fully replace a static random access memory or a dynamic random access memory. Also, another problem to be solved is that fine working of a ferroelectric capacitor is difficult.
Attention is paid to a magnetic memory called MRAM (Magnetic Random Access Memory) as a nonvolatile memory that does not have the drawbacks of the FRAM. MRAMs were initially based on a spin valve that makes use of an AMR (Anisotropic Magneto Resistive) effect (refer to, for example, J. M. Daughton, “Thin Solid Films”, Vol. 216, pp. 162–168, 1992 (hereinafter referred to as Non-Patent Document 1)) or a GMR (Giant Magneto Resistance) effect (refer to, for example, D. D. Tang et al., “IEDM Technical Digest”, pp. 997–999, 1995 (hereinafter referred to as Non-Patent Document 2)). However, the MRAMs of the types described have a drawback that, since the memory cell resistance of the load is as low as 10 Ω to 100 Ω, the power consumption per bit upon reading out is high and it is difficult to increase the capacity.
Meanwhile, the TMR (Tunnel Magneto Resistance) effect exhibits a rate of change in resistance of only 1% to 2% (refer to, for example, R. Meservey et al., “Physics Reports”, Vol. 238, pp. 214–217, 1994 (hereinafter referred to as Non-Patent Document 3)). However, in recent years, a rate of change in resistance of approximately 20% has been obtained (refer to, for example, T. Miyazaki et al., “J. Magnetism & Magnetic Material”, Vol. 139, L231, 1995 (hereinafter referred to as Non-Patent Document 4)), and attention has been paid to an MRAM which makes use of the TMR effect.
Since the MRAM is simple in structure, it is easy to achieve high integration, and since rotation of the magnetic moment is utilized for recording, it is estimated that the number of times by which it can be rewritten is great. Also, it is estimated that the MRAM allows very high speed accessing, and it has been reported already that the MRAM can operate at 100 MHz (refer to, for example, R. Scheuerlein et al., “ISSCC Digest of Technical Papers”, pp. 128–129, February 2000 (hereinafter referred to as Non-Patent Document 5)).
While the MRAM has the advantage that it is easy to raise the speed and achieve high integration as described above, it has the following problems. A conventional MRAM is formed from a memory array of a single MTJ shape. In order to achieve high thermal stability to achieve the storage retaining time as long as 10 years, it is necessary to design the MTJ element of the MRAM so as to have a high antimagnetic field. Also, in an MRAM memory chip in which a functioning block, which retains information for a comparatively short period of time and operates at a high speed, and another functioning block, which must retain information for a long period of time although it is accessed less frequently, are provided in a mixed manner, since the switching magnetic field is set to a comparatively high level for all of the functioning blocks so that at least retention of information for a long period of time may be assured. Therefore, the MRAM memory has a problem that the overall power consumption upon writing is high.